JP3265011B2 - Thin film waveguide type optical frequency shifter and optical integrated circuit chip for optical frequency shifter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical frequency shifter used in a precision measuring device or the like to which heterodyne interference is applied, and more particularly to an optical integrated circuit type optical frequency shifter constituted by a thin film optical waveguide.

[0002]

2. Description of the Related Art In a technique such as heterodyne interference measurement applied to a precision measuring apparatus, an optical frequency shifter is a very important element. An optical frequency shifter is a device that shifts the frequency of light by a certain amount (for example, from ω to ω + Δω), and causes the laser light whose frequency is shifted by the optical frequency shifter to interfere with the laser light of the original frequency. , Measurement by heterodyne interferometry, and the like. Above all, research on an ultra-precision position measuring apparatus to which heterodyne interferometry is applied has been advanced, and some of the measuring apparatuses have already been put into practical use. In such an optical frequency shifter,
Important characteristics are that the conversion efficiency, that is, the ratio of the intensity of the input light to the intensity of the output light shifted in frequency is large, and that the frequency shift amount can be selected in a wide range. Furthermore, like other optical components and optical elements, such an optical frequency shifter can be miniaturized and mass-produced, has a low driving voltage, is stable against vibrations, and has high reliability. The optical integrated circuit type device constituted by the above has been actively researched and developed.

The main types of such a thin film waveguide type optical frequency shifter are an electro-optical effect type optical frequency shifter using an electro-optical effect, and an acousto-optical effect type optical frequency shifter using an acousto-optical effect. There is. The electro-optic effect type optical frequency shifter shifts the frequency by an electro-optic effect by applying an electric field that changes in a sawtooth shape to a pair of electrodes provided on both sides of a thin-film optical waveguide. The acousto-optic effect type optical frequency shifter shifts the frequency by an amount corresponding to the frequency of the surface acoustic wave by crossing the surface acoustic wave with the guided light of the thin-film optical waveguide. The electro-optic effect type optical frequency shifter has advantages in that, in principle, the conversion efficiency can be increased, and a practically important frequency shift of several tens of kHz can be realized by one shifter. On the other hand, the acousto-optic effect type optical frequency shifter has a feature that the configuration is simple and a large frequency shift of several tens of MHz can be realized.

[0004] Comparing the two types of optical frequency shifters,
The electro-optic effect type optical frequency shifter is more practical in that high conversion efficiency can be obtained and a frequency shift of several tens of kHz can be realized by one element. Such a thin film waveguide type electro-optic effect type optical frequency shifter is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-2921.
No. 10 discloses an invention of a waveguide type optical frequency shifter. In the waveguide type optical frequency shifter described in this publication, a deflection mode splitter for branching guided light into two waveguides, an electrically controlled optical frequency shifter provided in one waveguide, and two waveguides are provided. A multiplexing element for merging the wave paths. Thus, the optical element for the heterodyne light source incorporating the optical frequency shifter can be configured as a thin film optical waveguide type element. Therefore, the optical element for a heterodyne light source can be miniaturized and mass-produced, and has high stability and high reliability.

[0005]

In the waveguide type optical frequency shifter described in the above publication, the electrically controlled optical frequency shifter is constituted by providing a pair of electrodes on both sides of the waveguide. Therefore, in order to shift the optical frequency by the electrically controlled optical frequency shifter, it is necessary to apply an electric field that changes in a sawtooth shape to the pair of electrodes. However, in order to change the applied electric field into a saw-tooth shape, the fall time must be as close to zero as possible, and there is a limit in changing the applied electric field sharply in the saw-tooth shape. Therefore, the frequency shift amount is several MHz at the maximum.
Limited to a degree. Furthermore, when the sawtooth shape of the applied electric field falls, there is a problem that the optical frequency of the output light temporarily drops suddenly.

On the other hand, in the case of the acousto-optic type optical frequency shifter, since the diffraction of light by the acousto-optic effect is used, the conversion efficiency is limited in principle. That is, since the diffraction efficiency is limited by the acousto-optic effect, 10 to 2
The conversion efficiency of about 0% is the limit. In the case of the acousto-optic type, since mode conversion occurs simultaneously with the shift of the optical frequency, a mode converter for returning the mode after the frequency shift is required. As a result, the structure of the optical frequency shifter becomes complicated and large. Thus, it has been extremely difficult to realize a thin film waveguide type optical frequency shifter that satisfies both high conversion efficiency and a wide range of frequency shift. Accordingly, an object of the present invention is to provide a thin-film waveguide type optical frequency shifter and an optical integrated circuit chip for the optical frequency shifter, which have a high conversion efficiency and a wide range of frequency shift amount.

[0007]

In order to solve the above-mentioned problems, a thin-film waveguide type optical frequency shifter according to the first aspect of the present invention is configured as follows. That is, this thin film waveguide type optical frequency shifter shifts the frequency ω of the input light L0 whose waveform is represented by cos (ωt) by −Δω at time t, as schematically shown in FIG. An optical frequency shifter, which divides the optical power of the input light L0 into four equal parts to form first, second, third and fourth guided lights L1 to L
4 and a first guided light L1.
A first thin-film waveguide-type optical phase modulator M1 that modulates the phase with a phase modulation waveform φ (t) and a second thin-film waveguide that modulates the phase of the second guided light L2 with a phase modulation waveform −φ (t). Waveguide-type optical phase modulator M2, thin-film waveguide-type optical phase shifter M5 for shifting the phases of third waveguide light L3 and fourth waveguide light L4 by-(π / 2), and thin-film waveguide optical phase shifter The third guided light L3 output from M5 is converted into a phase modulation waveform φ (t−
ΔT), a third thin-film waveguide-type optical phase modulator M3 that performs phase modulation with ΔT), and the fourth guided light L4 output from the thin-film waveguide-type optical phase shifter M5 with a phase modulation waveform −φ (t−ΔT). Fourth thin film waveguide type optical phase modulator M4 for phase modulation
And the first, second, third and fourth guided lights L1 to L4 output from the first, second, third and fourth thin film waveguide type optical phase modulators M1 to M4. And an optical multiplexer M6 for the output light L5. When the period of the phase modulation waveform φ (t) is 2T, φ (t) =
Δ (t) = 2ΔωT−Δ when Δωt, T ≦ t <2T
ωt, and Δω and T are Δω ·
T = mπ (where m is an integer), Δω and ΔT
Is Δω · ΔT = (1 + 4n) (π / 2) (where n
Is an integer), (corresponding to claim 1). Here, “the optical power of the input light L0 is divided into four equal parts”
This means that the first to fourth guided lights L1 to L4 are demultiplexed so that the optical powers thereof are substantially equal, and it is not always necessary to strictly and accurately divide the light into four.

In the thin film waveguide type optical frequency shifter according to the first aspect, an input optical fiber for inputting the input light to the optical demultiplexer, and the output light combined by the optical demultiplexer is output from the optical demultiplexer. Output optical fiber for output from the optical fiber, monitor means for monitoring a part or all of the first to fourth waveguide light output from the first to fourth thin film waveguide type optical phase modulators, and the monitor means For controlling the amount of phase modulation in the first, second, third or fourth thin film waveguide type optical phase modulator or the amount of phase shift in the thin film waveguide type optical phase shifter based on the monitor signal outputted from Alternatively, a configuration in which means are added can be adopted (corresponding to claim 2). Here, "monitoring a part of the first to fourth guided lights" means, for example, monitoring only the first guided light or only the second and fourth guided lights.
"Monitoring all of the first to fourth guided lights" means monitoring all of the first, second, third and fourth guided lights. Further, "monitoring" means extracting only a part of the optical power of each guided light.

According to a third aspect of the present invention, there is provided an optical integrated circuit chip for an optical frequency shifter comprising a thin-film optical waveguide, wherein the optical integrated circuit chip propagates through the thin-film optical waveguide. an optical demultiplexer for propagating divided into four equal parts the power of the guided light to the four branch waveguides, wherein
Provided near the first branch waveguide of the four branch waveguides
For the first phase modulation to which the modulation voltage φ (t) is applied.
An electrode and a second branch waveguide of the four branch waveguides
And a modulation voltage −φ (t) is applied near the
2 phase modulation electrodes and the four branch waveguides.
3 is provided near the branch waveguide, and the modulation voltage φ (t−Δ
T) a third phase modulation electrode to which T) is applied;
Provided near the fourth branch waveguide of the branch waveguides,
Fourth phase modulation to which a modulation voltage −φ (t−ΔT) is applied
Electrode, the third branch waveguide and the fourth branch waveguide
A third branch waveguide and a fourth branching waveguide.
Shifts the phase of guided light propagating through a branch waveguide by -π / 2
A phase shift electrode to which a modulation voltage to be applied is applied;
A first branch waveguide, the second branch waveguide, the third branch waveguide,
Combining the output of the branch waveguide and the output of the fourth branch waveguide
And the modulation voltage φ (t) is
When the period is 2T, φ (t) = when 0 ≦ t <T
Δ (t) = 2ΔωT−Δ when Δωt, T ≦ t <2T
ωt, and Δω and T are Δ
ω · T = mπ (where m is an integer).
ω and ΔT are Δω · ΔT = (1 + 4n) (π / 2) (
Where n is an integer).
Optical integrated circuit chip for shifters.

[0010]

The waveform cos (ω) of the input light in the thin-film waveguide type optical frequency shifter according to the present invention having the above-described configuration is now described.
Consider the change in t). In the following description, the amplitude intensity is assumed to be standardized. In the thin-film waveguide type optical frequency shifter of the present invention, the input light L0 whose waveform is represented by cos (ωt) at time t is divided into four equal parts in the optical demultiplexer M0, and the first light is divided into four. ,
The light is guided as second, third, and fourth guided lights L1 to L4. Then, the first thin film waveguide type optical phase modulator M
1, the first guided light L1 is phase-modulated by the phase modulation waveform φ (t), and the second thin film waveguide type optical phase modulator M
2 causes the second guided light L2 to have a phase modulation waveform −φ (t).
Is phase modulated. As a result, the waveform of the first guided light L1 becomes cos [ωt + φ (t)], and the waveform of the second guided light L2 becomes cos [ωt−φ (t)]. Accordingly, the sum (L1 + L2) of the waveforms of the first guided light L1 and the second guided light L2 is expressed by the following equation (1). L1 + L2 = cos [ωt + φ (t)] + cos [ωt−φ (t)] = 2 cos (ωt) · cos [φ (t)] (1)

Here, the phase modulation waveform φ (t) is described by the following equations (2) and (3), and has a triangular waveform as shown in FIG. When 0 ≦ t <T, φ (t) = Δωt (2) When T ≦ t <2T, φ (t) = 2ΔωT−Δωt (3) where 2T is the period of the phase modulation waveform φ (t). Is equivalent to That is, the function φ (t) expressed by equations (2) and (3)
Is repeated for time t. here,
Δω and T are set so as to satisfy the relationship of the following equation (4). ΔωT = mπ (where m is an integer) (4) Due to the nature of the cos function, the following equation (5) is obtained over the entire range of t of the variable φ (t) representing the phase modulation waveform.
Holds. cos [φ (t)] = cos (Δωt) (5) As a result, according to Expressions (1) and (5), the sum (L1 + L2) of the waveforms of the first guided light L1 and the second guided light L2 is obtained. Is represented by the following equation (6). L1 + L2 = 2 cos (ωt) cos (Δωt) (6)

On the other hand, in the third guided light L3 and the fourth guided light L4, first, the waveform cos (ωt) of the input light is shifted by-(π / 2) by the thin film waveguide type optical phase shifter M5. Then, the third guided light L3 output from the thin film waveguide type optical phase shifter M5 is phase-modulated by the third thin film waveguide type optical phase modulator M3 with a phase modulation waveform φ (t−ΔT), The fourth guided light L4 is phase-modulated by the fourth thin-film waveguide type optical phase modulator M4 with a phase modulation waveform -φ (t-ΔT). As a result, the waveform of the third guided light is
sin [ωt + φ (t−ΔT)], and the waveform of the fourth guided light is sin [ωt−φ (t−ΔT)]. Thus, the sum of the waveforms of the third guided light L3 and the fourth guided light L4 (L3 + L4)
Is given by the following equation (7). L3 + L4 = sin [[omega] t + [phi] (t- [Delta] T)] + sin [[omega] t- [phi] (t- [Delta] T)] = 2 sin ([omega] t) * cos [[phi] (t- [Delta] T)] (7) From (7), the following equation (8) is obtained. L3 + L4 = 2 sin (ωt) · cos [Δω (t−ΔT)] (8)

Here, the relationship between Δω and ΔT is expressed by the following equation (9).
Set as follows. Δω · ΔT = (1 + 4n) (π / 2) (n: integer) (9) As a result, according to the equations (8) and (9), the waveforms of the third guided light L3 and the guided light L4 of the guide 4 are obtained. The sum (L3 + L4) is given by the following equation (1)
0). L3 + L4 = 2 sin (ωt) · sin (Δωt) (10) Then, the first and second output from the first, second, third and fourth thin film waveguide type optical phase modulators M1 to M4. , Third
The fourth guided lights L1 to L4 are combined by the optical multiplexer M6 to be output light L5. Therefore, the output light L5
Is represented by the following equation (11) from the equations (6) and (10). L5 = L1 + L2 + L3 + L4 = 2 cos (ωt) cos (Δωt) +2 sin (ωt) sin (Δωt) = 2 cos [(ωt) − (Δωt)] = 2 cos [(ω−Δω) t] (11) )

As a result, as shown in equation (11), the waveform is cos (ω) with a conversion efficiency of 100% in theory.
The frequency ω of the input light represented by t) can be shifted by −Δω. Here, the frequency shift amount is −Δω, but Δω can be a negative value as long as the relationship of Expression (9) is satisfied, and the frequency shift amount −Δω can be a positive value. Further, the frequency shift amount −Δω is the phase modulation waveform φ
(t). This φ (t) has a triangular waveform as shown in FIG. 2 and has no problem as in the case of a sawtooth shape, and can stably oscillate from a low frequency of several Hz to a high frequency of several tens MHz or more. Is possible. Therefore,
The frequency shift amount-[Delta] [omega] of the thin film waveguide type optical frequency shifter according to the present invention can be selected in a very wide range from a low frequency of several Hz to a high frequency of several tens MHz or more. Thus, a thin film waveguide type optical frequency shifter having a high conversion efficiency and a wide range of frequency shift can be obtained.

In the thin film waveguide type optical frequency shifter according to the second aspect, the input light for inputting the input light to the optical demultiplexer in the thin film waveguide type optical frequency shifter according to the first aspect. A fiber, an output optical fiber for outputting the output light combined in the optical multiplexer from the optical multiplexer, and first to fourth thin film waveguide type optical phase modulators M1
To M4, the first to fourth guided lights L1 to L4
Monitoring means for monitoring a part or all of the first, second, and third monitoring means based on a monitoring signal output from the monitoring means.
Control means for controlling the amount of phase modulation in the third and fourth thin film waveguide type optical phase modulators or the amount of phase shift in the thin film waveguide type optical phase shifter is added. Therefore, the input of the light of the reference frequency to the thin-film waveguide type optical frequency shifter and the output of the light with the shifted optical frequency can be performed very easily and with high efficiency.
Further, the monitoring means controls the guided light L1 to L4 output from the first to fourth thin film waveguide type optical phase modulators M1 to M4.
By monitoring a part or all of L4 and feedback-controlling the first to fourth thin film waveguide type optical phase modulators M1 to M4 and the thin film waveguide type optical phase shifter M5, high conversion efficiency and stability are always achieved. The maintained frequency shift amount can be maintained. As a result, an extremely practical thin film waveguide type optical frequency shifter having high output, high stability in a wide band, and easy mounting can be obtained.

Further, in the optical integrated circuit chip for an optical frequency shifter according to the third aspect, the optical power of the guided light propagating through the thin-film optical waveguide is equal to that of the four branched waveguides by the optical demultiplexer. Propagated separately. A first phase modulation electrode is provided near the first branch waveguide among the four branch waveguides, and a second phase modulation electrode is provided near the second branch waveguide. Are provided, and a third phase modulation electrode is provided near the third branch waveguide, and a fourth phase modulation electrode is provided near the fourth branch waveguide. Further, a phase shift electrode is provided near the third branch waveguide and the fourth branch waveguide. Then, the outputs of the first branch waveguide, the second branch waveguide, the third branch waveguide, and the fourth branch waveguide are combined by the multiplexer. Therefore, the waveform φ is applied to the first phase modulation electrode.
(t), a phase modulation voltage of waveform −φ (t) is applied to the second phase modulation electrode, and the phase φ (t−ΔT) is applied to the third phase modulation electrode. A modulation voltage is applied, a phase modulation voltage having a waveform of -φ (t-ΔT) is applied to the fourth phase modulation electrode, and a voltage for shifting the phase of the guided light by-(π / 2) to the phase shift electrode. By applying
The frequency ω of the input light whose waveform is represented by cos (ωt) is shifted by −Δω based on the functions and effects described by the above equations (1) to (11). In this way,
By using the optical integrated circuit chip for an optical frequency shifter according to the third aspect, a thin film waveguide type optical frequency shifter having a high conversion efficiency and a wide frequency shift range can be obtained.

[0017]

Next, an embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a diagram showing the overall configuration of one embodiment of the thin film waveguide type optical frequency shifter according to the present invention. The thin-film waveguide type optical frequency shifter 1 shown in FIG. 3 includes a thin-film waveguide type optical phase shifter 4, Mach-Zehnder type optical phase modulators 6 and 8 also formed of a thin film waveguide, and input laser light having a reference frequency. The optical input unit 10 propagates the light to the waveguide, and the optical output unit 20 emits the frequency-shifted guided light. Hereinafter, the configuration of each unit will be described in detail.

As shown in FIG. 3, in the optical frequency shifter 1 of this embodiment, a channel type (three-dimensional) Ti-diffused LiNbO _{3 is} placed on a Z-cut LiNbO ₃ (lithium niobate) single crystal substrate 2. Optical waveguides 11 to 19, 23
27 and 32, 36 are formed. Note that FIG.
Is the x-axis in the waveguide thickness direction and y in the waveguide width direction.
The axis and the propagation direction of the guided light are taken along the z-axis. Ti
The diffused LiNbO ₃ optical waveguides 11 to 19, 23 to 27 and 32, 36 are single-mode optical waveguides, and the thickness and the width of the waveguide are respectively set in the x-axis direction and the y-axis direction of the guided light. The mode is created to be single mode. The z-axis of the coordinate axis, which is the propagation direction of the guided light, matches the X-axis direction of the Z-cut LiNbO ₃ single crystal substrate 2.

As shown in FIG. 3, the optical frequency shifter 1
The channel type optical waveguides 11 to 19 and 23 to 27 have three branch portions 12, 14, and 24 and three junction portions 16,
18 and 26. That is, one single-mode optical waveguide 11 is symmetrically branched into two single-mode optical waveguides 13 and 23 at the first branch portion 12.
The single mode optical waveguide 13 further includes a second branch portion 14
The two single-mode optical waveguides 15A and 15B
The single-mode optical waveguide 23 branches into two single-mode optical waveguides 25A and 25B at the third branch portion 24. Of these branched optical waveguides, the single-mode optical waveguides 15A and 15B merge at the first junction 16 to become the single-mode optical waveguide 17,
The single mode optical waveguides 25A and 25B are
At each other to form a single-mode optical waveguide 27. These single-mode optical waveguides 17 and 27 join at the third junction 18 to form a single-mode optical waveguide 19. It should be noted that a channel type T having the above branch
i-diffusion LiNbO ₃ optical waveguides 11 to 19, 23 to 27,
The creation of the monitoring optical waveguides 32 and 36 described later is as follows.
This is performed by a patterning process using a normal photolithography technique and a process of thermally diffusing Ti metal into the LiNbO ₃ single crystal substrate 2. Since these creating techniques are known techniques, a detailed description thereof will be omitted.

In the vicinity of the single mode optical waveguide 23, a pair of voltage applying electrodes 40A and 40B opposed to each other with the optical waveguide 23 interposed therebetween is provided. The voltage application electrodes 40A and 40B are formed by patterning an aluminum metal thin film, and are formed by sputtering aluminum metal on the LiNbO ₃ single crystal substrate 2 and photolithography. A pair of voltage application lead wires 41A and 41B are connected to the voltage application electrodes 40A and 40B by soldering. Voltage application lead wires 41A, 41
A power supply 42A and a ground 42B for applying a modulation voltage of V1 are connected to B, respectively. The above single mode optical waveguide 23, a pair of voltage applying electrodes 40A and 40B, voltage applying leads 41A and 41B,
The thin film waveguide type optical phase shifter 4 is constituted by the power source 42A and the earth 42B.

In the vicinity of the pair of branched single-mode optical waveguides 15A and 15B, the optical waveguides 15A and
5B, a pair of voltage applying electrodes 44A, 44
4B is provided. These voltage application electrodes 44A, 4
4B is formed by patterning an aluminum metal thin film similarly to the voltage application electrodes 40A and 40B. A pair of voltage application leads 45A and 45B are connected to the voltage application electrodes 44A and 44B, respectively.
A power supply 46A for applying a modulation voltage of V2 and a ground 46B are connected to the lead wires 45A and 45B, respectively. The above single mode optical waveguides 15A, 15B,
A pair of voltage application electrodes 44A and 44B, voltage application leads 45A and 45B, power supply 46A and ground 46B constitute a Mach-Zehnder optical phase modulator 6.

Here, when a modulation voltage of V2 is applied to the power supply 46A, the guided light of the single mode optical waveguide 15A becomes φ
If the phase modulation of (t) is received, the guided light of the single mode optical waveguide 15B is subjected to the phase modulation of -φ (t). That is, the Mach-Zehnder type optical phase modulator 6 comprises a first thin-film waveguide type optical phase modulator 6A for phase-modulating the guided light of the single-mode optical waveguide 15A, and a single-mode optical phase modulator having a waveform whose polarity is reversed. Second phase modulation of the guided light of the optical waveguide 15B
And the thin film waveguide type optical phase modulator 6B.

Further, a pair of single mode optical waveguides 25A
Similarly, a pair of voltage application electrodes 48A and 48B are provided near the first and second electrodes 25B. A pair of voltage application lead wires 49 is connected to the voltage application electrodes 48A and 48B.
A and 49B are connected to each other, and a power supply 50A and a ground 50B for applying a modulation voltage of V3 are connected to the respective lead wires 49A and 49B. The above single-mode optical waveguides 25A and 25B, a pair of voltage application electrodes 48A and 48B, and a voltage application lead wire 4
The 9A and 49B, the power supply 50A and the ground 50B constitute a Mach-Zehnder type optical phase modulator 8. The Mach-Zehnder type optical phase modulator 8 includes a third thin-film waveguide type optical phase modulator 8A for phase-modulating the guided light of the single-mode optical waveguide 25A, and a single-mode optical phase modulator having a waveform whose polarity is reversed. It can be divided into a fourth thin-film waveguide type optical phase modulator 8B for phase-modulating the guided light of the optical waveguide 25B.

As shown in FIG. 3, near the single-mode optical waveguides 17 and 27, single-mode channel optical waveguides 32 and 36 are provided, respectively. The single-mode optical waveguides 17 and 32 and the single-mode optical waveguides 27 and 36 each constitute a directional coupler. Therefore, a part of the guided light propagating through the optical waveguides 17 and 27 flows into the optical waveguides 32 and 36. The optical waveguides 32 and 36 are optical waveguides for monitoring the guided light propagating through the optical waveguides 17 and 27, and the monitor light propagating through the monitoring optical waveguides 32 and 36 are respectively supplied to the monitor light output unit 34 and the monitor light output unit 34. 38.

The thin-film waveguide type optical phase shifter 4
Changes the phase of the guided light of the single mode optical waveguide 23 to (−π) by applying the modulation voltage V1 by the power supply 42A.
/ 2). Further, when the modulation voltage of V2 is applied by the power supply 46A, the single-mode optical waveguide 1 in the first thin-film waveguide-type optical phase modulator 6A is applied.
The phase of the guided light of 5A is phase-modulated with the waveform φ ₁ (t), and at the same time, the phase of the guided light of the single-mode optical waveguide 15B is changed in the second thin-film waveguide type optical phase modulator 6B.
Phase modulated by φ ₁ (t). Further, when the V3 modulation voltage is applied by the power supply 50A, the phase of the guided light of the single mode optical waveguide 25A is changed to the waveform φ ₁ (t−T) in the third thin film waveguide type optical phase modulator 8A. / 4), and at the same time, the fourth thin-film waveguide type optical phase modulator 8B
In, the phase of the guided light in the single-mode optical waveguide 25B is phase-modulated with the waveform −φ ₁ (t−T / 4).

In the thin film waveguide type optical frequency shifter 1 configured as described above, how the waveform cos (ωt) of the input light changes will be considered. The phase modulation waveform φ ₁ (t) is described by the following equations (12) and (13). When 0 ≦ t <T, φ ₁ (t) = (2π / T) × t (12) When T ≦ t <2T, φ ₁ (t) = 4π− (2π / T) × t (13) Here, 2T is the period of the phase modulation waveform φ ₁ (t). That is, Expressions (12) and (13) are replaced by Expressions (2),
This corresponds to the case where Δω = (2π / T) in (3) or the case where m = 2 in equation (4). Therefore,
φ ₁ (t) also has a triangular waveform as shown in FIG. 2, and at this time, ΔωT = 2π in FIG. Then, from the property of the cos function, the following equation (14) is established, similarly to the equation (5). cos [φ ₁ (t)] = cos [(2π / T) × t] (14)

Therefore, each single mode optical waveguide 11 to 1
Output waveforms of the guided light in 9, 23 to 27 are as follows. First, the waveform of the guided light of the optical waveguide 15A is co
s [ωt + φ ₁ (t)], and the waveform of the guided light in the optical waveguide 15B is cos [ωt−φ ₁ (t)]. As a result, the sum of the waveforms of the guided lights of the optical waveguides 15A and 15B, that is, the guided light L17 that merges at the first junction 16 and propagates through the optical waveguide 17
Is as shown in the following equation (15). L17 = cos [ωt + φ ₁ (t)] + cos [ωt−φ ₁ (t)] = 2 cos (ωt) · cos [φ ₁ (t)] (15) Accordingly, from the expression (14), the optical waveguide 17 is obtained. Is represented by the following equation (16). L17 = 2 cos (ωt) ・ cos [(2π / T) × t] (16)

The output waveform of the guided light of the optical waveguide 25A is sin [ωt + φ ₁ (t−T / 4)], and the optical waveguide 25B
The output waveform of the guided light is sin [ωt−φ ₁ (t−T / 4)]. This is because ΔT = T / 4 in equation (7) described above.
Is equivalent to Therefore, the optical waveguides 25A and 25A
The sum of the outputs of the guided lights B, that is, the output waveform of the guided light L27 that is merged at the second merging portion 26 and propagates through the optical waveguide 27 is given by the following equation (17) from equations (7) and (14). Become like L27 = sin [ωt + φ ₁ (t−T / 4)] + sin [ωt−φ ₁ (t−T / 4)] = 2 sin (ωt) · cos [φ ₁ (t−T / 4)] = 2 sin (ωt) · cos [(2π / T) × (t−T / 4)] (17) As a result, the waveform of the guided light L27 propagating through the optical waveguide 27 is expressed by the following equation (18). L27 = 2 sin (ωt) · cos [(2π / T) × t−π / 2] = 2 sin (ωt) · sin [(2π / T) × t] (18)

Therefore, the sum (L17 + L27) of the outputs of the guided lights of the optical waveguides 17 and 27, that is, the output waveform of the guided light L19 which is joined at the third junction 18 and propagates through the optical waveguide 19 is expressed by the following equation (16). From the equation (18), the following equation (19) is obtained. L19 = L17 + L27 = 2 cos (ωt) cos [(2π / T) × t] +2 sin (ωt) sin [(2π / T) × t] = 2 cos ｛(ωt) − [(2π / T) × t]｝ = 2 cos ｛[ω− (2π / T)] t｝ (19) As shown in Expression (19), the waveform of the guided light L19 propagating through the optical waveguide 19 is cos (ωt) From the input waveform of
The output waveform is converted to {[ω− (2π / T)] t}. That is, the optical frequency is shifted by-(2π / T) at a conversion efficiency of 100% in theory.

Note that Δω · ΔT = (2π / T) (T / 4) = π / 2
Where Δω and ΔT satisfy the relationship of the above equation (9). Further, the frequency shift amount −Δω is the phase modulation waveform φ
₁ (t), φ ₁ (t) is a triangular waveform as shown in FIG. 2 and can be transmitted stably from a low frequency to a high frequency.
ω can be selected in an extremely wide range from several Hz to high frequencies of several tens of MHz or more. Thus, a thin film waveguide type optical frequency shifter having a high conversion efficiency and a wide range of frequency shift can be obtained.

Next, an element in which the above-described thin film waveguide type optical frequency shifter 1 is unitized will be described with reference to FIG. FIG. 4 is a diagram showing an overall configuration of an optical frequency shifter unit 70 in which the thin-film waveguide type optical frequency shifter 1 is incorporated. As shown in FIG. 4, the optical frequency shifter unit 70 includes an input optical fiber 7 for inputting input light to the optical input unit 10 of the thin film waveguide type optical frequency shifter 1.
4. It has an output optical fiber 84 for outputting the light whose frequency is shifted from the optical output unit 20, and a monitor signal line 78 for monitoring the monitor light output from the monitor light output units 34 and 38. .

A monitor signal is processed on the monitor signal line 78, and the thin film waveguide type optical phase shifter 4 and the first thin film waveguide type optical phase modulator 6 are processed based on the processing result.
A, The amount of phase shift or the amount of phase modulation in the second thin film waveguide type optical phase modulator 8A and the fourth thin film waveguide type optical phase modulator 8B is fed back. Signal processing unit 8 for controlling
0 is connected. From the signal processing unit 80, a control signal line 82 is connected to the power supplies 42A, 46A, 50A of the thin-film waveguide optical frequency shifter 1. The signal processing unit 80 includes a CPU (central processing unit) and an RA.
This is a computer system mainly configured with M and ROM memory devices. Then, the waveform represented by the monitor signal converted into the electric signal is converted to the single mode optical waveguides 17 and 2.
7 are compared with target waveforms of the guided lights L17 and L27 propagating through the control signal line 7, and the result is sent to the control signal line 82 as a control signal. Thus, the voltages of the power supplies 42A, 46A, and 50A of the thin-film waveguide optical frequency shifter 1 are feedback-controlled. The monitor signal line 78 may be an optical fiber or the like that propagates the monitor light output from the monitor optical waveguides 32 and 36 as it is, or converts the monitor light into an electric signal by a photodetector and transmits the electric signal. It may be a cable. In the former case, the signal processing unit 8
A light detector will be provided on the 0 side.

As described above, in the optical frequency shifter unit 70 of the present embodiment, the input optical fiber 74, the output optical fiber 84,
A monitor signal line 78 for monitoring monitor light output from the monitor optical waveguides 32 and 36 and a signal processing unit 80 are added. Thereby, the input of the input laser light from the input end 72 of the input optical fiber 74 to the thin-film optical waveguide type optical frequency shifter 1 and the output of the frequency-shifted laser light from the output end 86 of the output optical fiber 84. The output can be performed very easily and with high efficiency. Furthermore, by monitoring the guided light by the monitoring optical waveguides 32 and 36 and performing feedback control, it is possible to always maintain high conversion efficiency and a stable frequency shift amount. As a result, an extremely practical thin film waveguide type optical frequency shifter having high output, high stability in a wide band, and easy mounting can be obtained.

In this embodiment, the first and second thin film waveguide type optical phase modulators 6A and 6B, and the third and fourth thin film waveguide type optical phase modulators 8A and 8B are paired, respectively. Although the case where the optical phase modulators are configured as Mach-Zehnder type optical phase modulators 6 and 8 has been described, a pair of electrodes are provided on both sides of the first to fourth channel type optical waveguides 15A, 15B, 25A and 25B, and separate electrodes are provided. You may comprise as a thin film waveguide type optical phase modulator. Of course, only one of the pair of thin-film waveguide type optical phase modulators can be a Mach-Zehnder type optical phase modulator. Further, in the present embodiment, the case where the two-branch optical waveguides symmetrically branched from the single-mode optical waveguides 11, 13, and 23 are combined is shown. Alternatively, it is also possible to form a waveguide pattern in which the light is first branched into three and then one of them is branched into two. Further, the branching structure of the optical waveguide may be any as long as it finally divides the optical power of the input light into four, and the branching does not necessarily have to be symmetric.

In this embodiment, the Z-cut Li
Although the case where an X-direction propagating single mode optical waveguide is formed on the NbO ₃ single crystal substrate is shown, the LiNbO ₃ single crystal substrate may be X-cut or Y-cut, and the propagation direction of the guided light may be set variously. Can be. LiNbO ₃
As a method of forming an optical waveguide on a single crystal substrate, a method such as a proton exchange method may be used in addition to the Ti diffusion method described above. Furthermore, if a waveguide-type element that performs phase modulation of guided light by an electro-optic effect can be configured, a material such as an optical single crystal other than LiNbO ₃ can be selected as a substrate, and a method of forming a waveguide can be used as a substrate material. It can be appropriately selected in accordance with this. The material constituting the electrode may be a metal other than aluminum. In the present embodiment, the channel-type optical waveguides 17 and 27 and the linear channel-type optical waveguide 3 constituting the directional coupler respectively.
Reference numerals 2 and 36 are provided for monitoring the guided light, and are not essential components of the present invention. The structure, shape, size, material, number, arrangement, and the like of other portions of the thin film waveguide type optical frequency shifter are not limited to the above embodiments.

[0036]

According to the present invention, a plurality of thin-film waveguide type optical phase modulators utilizing the electro-optic effect and a thin-film waveguide type optical phase shifter are combined so that each phase modulation waveform and the amount of phase shift are kept constant. Since the set thin film waveguide type optical frequency shifter is created, a thin film waveguide type optical frequency shifter having high conversion efficiency and a wide selection range of the frequency shift amount can be obtained. As a result, an extremely practical thin film waveguide type optical frequency shifter having high performance, high reliability, small size, and excellent mass productivity is obtained. Since an arbitrary frequency shift amount can be set, it can be applied not only to the field of optical measurement but also to a wide range of fields such as optical communication and optical information processing.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a thin film waveguide type optical frequency shifter according to the present invention.

FIG. 2 is a diagram showing a phase modulation waveform in the thin film waveguide type optical frequency shifter according to the present invention.

FIG. 3 is a diagram showing an overall configuration of an embodiment of a thin film waveguide type optical frequency shifter according to the present invention.

FIG. 4 is a diagram showing an entire configuration of an optical frequency shifter unit to which an embodiment of a thin film waveguide type optical frequency shifter is applied.

[Explanation of symbols]

Reference Signs List 1 thin-film waveguide type optical frequency shifter L0 input light L1 first guided light L2 second guided light L3 third guided light L4 fourth guided light L5 output light M0, 10 to 14, 15A, 15B, 23, 24, 25
A, 25B Optical demultiplexer M1, 6A First thin-film waveguide optical phase modulator M2, 6B Second thin-film waveguide optical phase modulator M3, 8A Third thin-film waveguide optical phase modulator M4 , 8B Fourth thin film waveguide type optical phase modulator M5,4 Thin film waveguide type optical phase shifter M6, 15A, 15B, 16-20, 25A, 25B,
26,27 Optical multiplexer

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-27820 (JP, A) Shuichiro Asakawa et al., Proc. Of the 1993 IEICE Spring Conference, March 15, 1993, Volume 4, Page 4-241 K. Wong and R.S. M. De La Rue, Optics Letters, Vol. 7, No. 11 (1982), p. 546-548 Tsuyoshi Yamazaki et al., IEICE Technical Report, October 30, 1987, Vol. 87, No. 232, pp. 29-36 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/01-2/02 JICST file (JOIS) WPI (DIALOG)

Claims (3)

(57) [Claims] 1. An optical frequency shifter for shifting a frequency ω of input light having a waveform represented by cos (ωt) by −Δω for a time t, wherein the optical power of the input light is divided into four equal parts, and An optical demultiplexer that guides the first, second, third, and fourth guided lights as light, and a first thin-film waveguide-type optical phase modulator, which performs phase modulation of the first guided light with a phase modulation waveform φ (t). A second thin-film waveguide-type optical phase modulator that modulates the phase of the second guided light with a phase modulation waveform -φ (t); and adjusts the phases of the third guided light and the fourth guided light. − (Π /
2) a thin-film waveguide-type optical phase shifter shifted by only 2); and a third thin-film waveguide for phase-modulating the third guided light output from the thin-film waveguide-type optical phase shifter with a phase modulation waveform φ (t-ΔT). Optical phase modulator; and the fourth optical phase modulator output from the thin-film waveguide optical phase shifter.
A fourth thin-film waveguide-type optical phase modulator for phase-modulating the guided light with a phase modulation waveform -φ (t-ΔT); and the first, second, third, and fourth thin-film waveguide-type optical phases. The first, second, third and fourth signals output from the modulator;
And an optical multiplexer that combines the guided light beams and outputs the combined light beam. The phase modulation waveform φ (t) has a period of 2T, and when 0 ≦ t <T, φ (t) = Δωt, T ≦ t <2
T is a triangular waveform represented by φ (t) = 2ΔωT−Δωt, where Δω and T are Δω · T = mπ (where m is an integer)
), And Δω and ΔT are Δω · ΔT = (1 + 4n) (π /
2) (where n is an integer). 2. An input optical fiber for inputting the input light to the optical demultiplexer, an output optical fiber for outputting output light combined in the optical multiplexer from the optical multiplexer, Monitoring means for monitoring a part or all of the first to fourth guided light outputted from the first to fourth thin film waveguide type optical phase modulators, based on a monitor signal outputted from the monitoring means; The first, second, third
2. The apparatus according to claim 1, further comprising control means for controlling a phase modulation amount in the fourth thin film waveguide type optical phase modulator or a phase shift amount in the thin film waveguide type optical phase shifter. A thin film waveguide type optical frequency shifter. 3. An optical frequency constituted by a thin film optical waveguide.
An optical integrated circuit chip for a shifter, wherein the power of guided light propagating through the thin-film optical waveguide is divided by four.
An optical demultiplexer that divides the light into four equal parts and propagates the light into the branch waveguide, and an optical splitter near the first branch waveguide among the four branch waveguides.
And a first phase shifter to which a modulation voltage φ (t) is applied.
A tuning electrode and a portion near the second branch waveguide among the four branch waveguides.
A second phase to which the modulation voltage −φ (t) is applied
A modulating electrode, and a modulating electrode , near the third branch waveguide of the four branch waveguides.
A third voltage to which the modulation voltage φ (t−ΔT) is applied.
A phase modulation electrode and a fourth branch waveguide in the vicinity of the fourth branch waveguide.
And a fourth voltage to which the modulation voltage −φ (t−ΔT) is applied.
In the vicinity of the third branch waveguide and the fourth branch waveguide.
And a third branch waveguide and a fourth branch waveguide are provided.
Modulation that shifts the phase of propagating guided light by -π / 2
A phase shift electrode to which a voltage is applied, the first branch waveguide, the second branch waveguide, and the
3 and the output of the fourth branch waveguide.
The modulation voltage φ (t) has a period of 2T,
When 0 ≦ t <T, φ (t) = Δωt, and T ≦ t <2T
Triangular wave represented by φ (t) = 2ΔωT−Δωt
In the form, the [Delta] [omega and T, Δω · T = mπ (although, m is an integer
), And Δω and ΔT are Δω · ΔT = (1 + 4n) (π /
2) (where n is an integer)
Optical integrated circuit chip for optical frequency shifter.